The invention relates generally to voltage controlled oscillators, and more particularly to voltage controlled oscillators suitable for applications at frequencies of 3 GHz and higher.
A voltage controlled oscillator (VCO) is a critical component in almost every digital communications systems. VCOs generate an oscillating signal at a frequency determined by an external voltage, which is useful for tracking and matching signal frequencies. In a digital communication system, VCOs are used for clock generation and synchronization to transmit and receive data.
As is well known in the field of circuit design, there are various types of VCOs. The particular type of VCOs of interest herein is the type of VCOs that utilize ring oscillators. A ring oscillator includes a number of inverters or differential delay cells that are connected in series. The output of the last differential delay cell is typically connected to the first differential delay cell and consequently, the differential delay cells of the oscillator form a circular path. The frequency at which the VCO oscillates is primarily determined by the delays caused by the differential delay cells included in the ring oscillator.
A conventional VCO of interest is described in U.S. Pat. No. 5,936,476 to Iravani. In one embodiment, the VCO of Iravani includes an amplifier, a replica circuit and a number of VCO cells. Each VCO cell includes a pair of differential transistors that are connected to a common current source. The differential transistors are both connected to a supply voltage (VDD) via a source follower transistor. The replica circuit includes a single xe2x80x9cdifferentialxe2x80x9d transistor that is connected to a current source and to VDD via a source follower transistor. The replica circuit is designed to match the current and voltage characteristics of the VCO cells. A negative feedback loop connects the replica circuit to the amplifier of the VCO. The amplifier is configured to receive an external voltage and the negative feedback voltage from the replica circuit. The output of the amplifier is connected to the gates of the source follower transistors that are connected to the replica circuit and the VCO cells. The amplifier operates to control each of the source follower transistors in the same manner, and thus, each of the VCO cells compensates for any perturbation in VDD. The VCO of Iravani is described as having an operating frequency of 3 GHz and higher.
Another VCO of interest is described in U.S. Pat. No. 6,037,842 to Bryan et al. The VCO of Bryan et al. includes a differential control voltage, a plurality of variable delay elements, a buffer circuit and a differential driver circuit. The variable delay elements are connected in a ring oscillator configuration. Each variable delay element includes a pair of parallel connected differential CMOS sections, which are controlled by a differential control voltage. The differential control voltage magnitude sets the relative level of operation for the variable delay element, which determines the delay through that variable delay element. The VCO of Bryan et al. is described as having an operating frequency in the GHz range.
Although the above-described VCOs and other known VCOs operate well for their intended purposes, there is a growing interest to develop a VCO suitable for applications at frequencies of 3 GHz and higher that has lower voltage and power requirements, as well as fewer electrical components. In view of this interest, what is needed is a VCO circuit suitable for applications at frequencies of 3 GHz and higher that includes a small number of electrical components and requires low operating voltage and power.
A voltage controlled oscillator (VCO) and method for generating voltage controlled oscillating signals utilizes a plurality of timing blocks that form a circular signal path to provide delays to control the frequencies of the signals. The VCO is suitable for applications at frequencies of 3 GHz and higher, and has low voltage and power requirements. The delays provided by the timing blocks are determined by the operating properties of differential transistors, timing capacitors and current sources, which are included in the timing blocks.
A VCO in accordance with the present invention includes a number of timing blocks that are connected in a circular signal path configuration. Each timing block includes a differential voltage gain circuit that is configured to output amplified differential signals in response to input differential signals, and a frequency control circuit that is coupled to the differential voltage gain circuit. The frequency control circuit includes a first transistor and a first current source on a first current path, and a second transistor and a second current source on a second current path. The first current path has a first output terminal, which is connected to a first capacitor. Similarly, the second current path has a second output terminal, which is connected to a second capacitor. The first and second transistors are controlled by the amplified differential signals, while the first and second current sources are controlled by an input voltage to source defined amounts of current. The defined amounts of current at least partially define frequencies of output differential signals on the first and second output terminals.
In an exemplary embodiment, the VCO includes a first timing block and a second timing block. Each of the first and second timing blocks includes input terminals and output terminals. The first and second timing blocks are configured such that the output terminals of the first timing block are connected to the input terminals of the second timing block, and the output terminals of the second timing block are connected to the input terminals of the first timing block. In an embodiment, the input terminals and the output terminals of the first and second timing blocks are connected such that output differential signals from the first timing block are inversely received by the second timing block.
In one embodiment, the first and second transistors of the frequency control circuit are bipolar transistors. In another embodiment, the first and second transistors of the frequency control circuit are metal oxide semiconductor transistors. In either embodiment, each of the first and current sources of the frequency control circuit may include a signal metal oxide semiconductor transistor.
The differential voltage gain circuit of each timing block may include one or more differential amplifiers. Each differential amplifier includes differential transistors that are connected to a common current source. In an embodiment, each differential transistor is connected to a resistor, which is connected to a supply voltage.
A method for generating voltage controlled oscillating signals in accordance with the invention includes the steps of receiving first differential signals, amplifying the first differential signals to generate amplified first differential signals, applying the amplified second differential signals to parallel transistors, controlling amounts of current sourced by current sources that are connected to the parallel transistors using an input voltage, and outputting second differential signals having predefined frequencies on output terminals that are connected to the parallel transistors. The output terminals are also connected to capacitors. The predefined frequencies of the second differential signals are at least partially determined by the amounts of current sourced by the current sources and the capacitances of the capacitors. In an embodiment, the first differential signals and the second differential signals have a quadrature phase relationship.
The method may further include the steps of receiving the second differential signals, amplifying the second differential signals to generate amplified second differential signals, applying the amplified second differential signals to second parallel transistors, controlling amounts of current sourced by second current sources that are connected to the second parallel transistors using the input voltage, and outputting the first differential signals having the predefined frequencies on second output terminals that are connected to the second parallel transistors. The second output terminals are connected to second capacitors.
In an embodiment, the step of controlling the amounts of current sourced by the current sources includes supplying the input voltage to parallel current control transistors that function as the current sources. Furthermore, the step of amplifying the first differential signals may include amplifying the first differential signals in two stages using multiple differential amplifiers.
In an embodiment, the parallel transistors are bipolar transistors. In another embodiment, the parallel transistors are metal oxide semiconductors.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.